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Re: 8.1 -> 8.4 regression

From: Dennis Björklund <db(at)zigo(dot)dhs(dot)org>
To: "Ben Chobot" <bench(at)silentmedia(dot)com>
Cc: "PostgreSQL Performance" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: 8.1 -> 8.4 regression
Date: 2010-02-15 09:40:49
Message-ID: c50ac4425997cc51b9f371ae0214d90a.squirrel@zigo.org (view raw or flat)
Thread:
Lists: pgsql-performance
Can you force 8.4 to generate the same plan as 8.1? For example by running

  SET enable_hashjoin = off;

before you run EXPLAIN on the query? If so, then we can compare the
numbers from the forced plan with the old plan and maybe figure out why it
didn't use the same old plan in 8.4 as it did in 8.1.

Note that the solution is not to force the plan, but it can give us more
information.

/Dennis

> is at least one query which has degraded in performance quite a bit. Here
> is the plan on 8.4.2:
> http://wood.silentmedia.com/bench/842
>
> Here is the very much less compact plan for the same query on 8.1.19:
> http://wood.silentmedia.com/bench/8119



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